PHP custom class objects and POST - php

Question

I am trying to understand PHP login system using this php-login-advanced by panique. The code snippet for register.php is:
<?php
// check for minimum PHP version
if (version_compare(PHP_VERSION, '5.3.7', '<')) {
exit('Sorry, this script does not run on a PHP version smaller than 5.3.7 !');
} else if (version_compare(PHP_VERSION, '5.5.0', '<')) {
// if you are using PHP 5.3 or PHP 5.4 you have to include the password_api_compatibility_library.php
// (this library adds the PHP 5.5 password hashing functions to older versions of PHP)
require_once('libraries/password_compatibility_library.php');
}
// include the config
require_once('config/config.php');
// include the to-be-used language, english by default. feel free to translate your project and include something else
require_once('translations/en.php');
// include the PHPMailer library
require_once('libraries/PHPMailer.php');
// load the registration class
require_once('classes/Registration.php');
// create the registration object. when this object is created, it will do all registration stuff automatically
// so this single line handles the entire registration process.
$registration = new Registration();
// showing the register view (with the registration form, and messages/errors)
include("views/register.php");
and in class Registration the constructor is:
public function __construct()
{
session_start();
// if we have such a POST request, call the registerNewUser() method
if (isset($_POST["register"])) {
$this->registerNewUser($_POST['user_name'], $_POST['user_email'], $_POST['user_password_new'], $_POST['user_password_repeat'], $_POST["captcha"]);
// if we have such a GET request, call the verifyNewUser() method
} else if (isset($_GET["id"]) && isset($_GET["verification_code"])) {
$this->verifyNewUser($_GET["id"], $_GET["verification_code"]);
}
}
On line $registration = new Registration(); a new object of type Registration is created and it's constructor is called on Server-Side. The Server calls session_start() function. If a valid Session-ID was sent by client then file corresponding to that Session-ID is loaded in $_SESSION variable or else a new Session-ID is generated and is sent to client to store that in a COOKIE to maintain the session. The rendered register.php page is sent to client. Client fills form and submits. The values of form get stored in the $_POST array on Server-Side and this time when NEW object of class Registration is created the other two lines are also executed and the user is registered.
My Question is:
Am I understanding correctly the lifetime of $_POST, $_SESSION and the Registration Object? A better explanation is most welcome.